1. Field of the Invention
The present invention relates to bidirectional switches, currently called triacs, of medium power.
2. Discussion of the Related Art
Generally, a first surface, or rear surface of a triac is meant to be connected to a radiator (heat sink) and is covered with a uniform metallization forming a first main terminal of the triac. The opposite surface, or front surface, is covered with a second metallization of large dimensions forming the second main terminal of the triac, and one or more secondary metallizations, at least one of which forms a control terminal or gate of the triac.
Generally, the control terminal is referenced with respect to the second main terminal located on the same front surface. This can be a disadvantage, especially when it is desired to implement, monolithically or not, several triacs having a common main terminal (first or second) which is generally connected to ground that are also mounted on a radiator. In such an arrangement, the first main terminals (i.e., those on the rear surface) of the several triacs would be connected together and mounted on a common radiator connected to ground. The gates of the several triacs would then be controlled by a signal that is referenced to the second main terminals (i.e., those. on the front surface) which are at high and possibly different potentials. To selectively control the different triacs, a control circuit needs to be provided, the reference voltages of which are high and possibly distinct voltages. Thus, the problem of implementing relatively complex control circuits arises. Alternatively, discrete triacs, having their second main terminals (those on the front surface) commonly connected to ground and having their rear surfaces mounted on a radiator for cooling and being at different potentials (which results in the necessity of providing one radiator per triac or isolated assemblies) have to be used.
This situation appears in a great number of devices. For example, in a washing machine, several triacs are used for controlling the pumps, the solenoid-operated filling valves, the various distributors, etc. Such medium power triacs are controlled by the same programmer unit and their main terminals on which the gates are referenced are interconnected. Since these main terminals are disposed on the same side as the gate, it is not possible to implement a monolithic structure and braze the metallization corresponding to the common terminal on a same radiator, because the gate metallizations disposed on the same surface would then be short-circuited. Such configurations are to be found in many other systems, for example, in controls of rolling shutters in which the engines include a coil for the closing of the shutter and another coil for its opening.
Thus, an object of the present invention is to provide a triac structure such that several triacs can be assembled in a network with a common electrode, the common electrode forming the reference with respect to which the voltage applied to the control electrode is selected.
Another object of the present invention is to provide such a structure for alternating currents wherein the control voltage always has the same biasing with respect to the common reference main terminal, regardless of the biasing on the other main terminal (positive or negative halfwave of the mains voltage).
Another object of the present invention is to provide a monolithic structure incorporating a triac network.
Another object of the present invention is to provide such a monolithic structure which is easy to manufacture with currently used methods for manufacturing thyristors and triacs.
To achieve these objects, the present invention provides a triac network wherein each triac includes a semiconductor substrate of the first type of conductivity having a front surface and a rear surface, a layer of the second type of conductivity on the rear surface side, a deep diffusion of the second type of conductivity connecting the layer to the front surface, a first well of the second type of conductivity containing a first region of the first type of conductivity on the front surface side, a second well of the second type of conductivity on the front surface side, a second region of the first type of conductivity on the rear surface side substantially facing the second well, and a third well of the second type of conductivity containing a third region of the first type of conductivity on the front surface side. A first metallization on the rear surface side corresponds to a first main electrode, a second metallization on the front surface side covers the upper surface of the first region and of the second well and corresponds to a second main electrode, a third metallization covers one of the third well and the third region and is connected to a gate terminal, and a fourth metallization connects the other of the third well and the third region to the upper surface of the deep diffusion.
According to an embodiment of the present invention, all triacs are formed in a same semiconductor substrate and the first metallization is a common metallization covering the rear surface.
According to an embodiment of the present invention, the deep diffusion extends at the circumference of each triac structure.
According to an embodiment of the present invention, the first type of conductivity is type N.
According to an embodiment of the present invention, the control terminal is connected to the third region, and the network further includes means for applying to this control terminal a negative voltage with respect to the potential of the first main electrode.
According to an embodiment of the present invention, the control terminal is connected to the third well, and the network further includes means for applying to this control terminal a positive voltage with respect to the potential of the first main electrode.
These objects, characteristics and advantages as well as others, of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in relation with the accompanying drawings.